1. Field of the Invention
This invention relates to borehole measurements and more particularly to borehole measurements employing nuclear magnetic resonance.
2. Background of the Art
There are known in the patent literature various techniques for carrying out borehole measurements employing Nuclear Magnetic Resonance (NMR). In most such devices, a static magnetic field is used to align the magnetic spin of nucleii in the formation to in a preferential direction. Earlier devices used the earth's magnetic field as the static field while in recent years, static fields of greater strength have been produced by means of permanent magnets. A Radio Frequency (RF) transmitter is used to produce a magnetic field in the formation with its field direction orthogonal to the static magnetic field: this RF field causes a precession of the spins of the nucleii. Electromagnetic signals produced by this precession are detected by a receiver and analyzed to give information about the formation. The characteristics of this precession signal are related to the strength of the static magnetic field, the type of the nucleii and the frequency of the RF signal. One of the desirable characteristics of an NMR measuring device is to be able to produce a strong static field that is relatively uniform in the direction of tool travel over a substantial portion of the formation (to increase the signal-to-noise ratio and to reduce the effect of tool motion) and away from the borehole (to avoid signals from borehole fluids).
U.S. Pat. No. 4,350,955 discloses a device in which the static magnetic field is produced by a pair of permanent magnets with like poles opposing each other across a gap. The magnets have their magnetization axis along the longitudinal axis of the tool and borehole. The region of examination produced by this device is a toroid centered on the longitudinal axis and the gap between the permanent magnets. The static magnetic field within the region of examination is radial with respect to the longitudinal axis of the tool. The RF field is produced by a coil antenna with its axis parallel to the longitudinal axis and has a direction parallel to the longitudinal axis, producing the necessary orthogonality to the static field. This particular device suffers from a number of drawbacks. The region of examination is quite limited in the axial direction, so that the echo signals from the precession are weak. The device is susceptible to errors because of the motion of the tool. In addition, the static field also has a region within the borehole where the magnitude may be substantially the same as in the toroidal region, giving rise to a fairly strong echo signal from borehole fluids.
U.S. Pat. Nos. 5,212,447 and 5,280,243 disclose devices in which the region of examination is cylindrical. The static magnetic field is produced by cylindrical magnets with a magnetic axis perpendicular to the axis of the cylinder and the borehole. With a sufficiently long cylindrical magnet, the static field is approximately that of a line dipole and has a cylindrical region centered on the longitudinal axis in which the field is substantially constant. The direction of the static field may be radial, circumferential, or in between, depending upon the azimuthal position with respect to the magnet axis. The RF field is produced by a rectangular loop antenna with the plane of the rectangle parallel to the longitudinal axis of the borehole. This loop antenna effectively acts as a dipole that is orthogonal to the line dipole of the static field, so that the RF field within a large portion of the cylindrical region of examination is orthogonal to the static field. However, there will be portions where the two fields are not exactly orthogonal, effectively diminishing the size of the region of examination where orthogonality exists.
The essential difference between the devices of U.S. Pat. Nos. 5,212,447 and 5,280,243 is that the former is designed for wireline applications while the latter is designed for MWD applications. In both devices, by making the magnets longer, the region of examination can be extended, thus increasing the magnitude of the echo signal. The former device also has a gradient coil for reducing the effect of echo signals from the borehole fluids.
In both devices, the permanent magnet has to be made of a non-conducting material. This is necessary because the RF coil surrounds the permanent magnet and if the magnet were made conducting, induced currents in the magnet would reduce the efficiency of the tool. In addition, the presence of a conducting core inside the RF antenna severely distorts the RF magnetic field. Non-conducting permanent magnets are inherently not as powerful as conducting permanent magnets made of Samarium-Cobalt or of Neodymium-Iron. Consequently, to produce a given static magnetic field strength in the region of examination, devices using non-conducting magnets require a larger magnet size than comparable devices in which a conducting magnet may be used. In addition, the induced currents would also be detrimental to the efficiency of the RF antenna.
Furthermore, due to the fact that the static and magnetic fields are produced by line dipoles mechanically oriented along the axis of the tool (but with orthogonal magnetic orientation), known field enhancement techniques such as using ferrite cores or yokes cannot be used with the RF antenna of prior art devices: such a ferrite device would effectively short the static field. If ferrite yokes could be used as part of the RF antenna design, the antennas themselves could be made more compact to attain a given field strength and a given signal-to-noise ratio in operation.
There is a need for an NMR tool that takes advantage of the inherently higher magnetic fields attainable by the use of conducting permanent magnets. Such a device should also be able to obtain improved signal-to-noise ratio for the RF antenna. The present invention satisfies this need.